The term “bimodal” as applied to polyolefin resins usually means that the resin has two distinct ranges of molecular weight or density, which can impart desired properties to the product in great variety. Originally, bimodal resins were made in two separate reactors or reaction chambers—that is, a product having a first molecular weight was moved directly from the reaction zone in which it was made and introduced to a reaction zone having conditions for making a resin of a different molecular weight, where more resin was made. The two resins are thus mixed or, in some cases, even present in the same particles. Two-stage processes are difficult to control and, perhaps more important, have a capital disadvantage in that two reactors, or at least two reaction zones, are required to make them. Moreover, frequently the products are not homogeneously mixed in that at least some particles are entirely of one mode or the other. It is therefore desirable to find ways of making homogeneous bimodal polyolefins in a single reactor.
One approach to making bimodal polyolefins in a single reactor has been to employ a mixed catalyst system, in which one catalyst component makes a primarily low molecular weight (LMW) product and the other catalyst component produces a primarily high molecular weight (HMW) product because of different termination and/or chain transfer kinetics. By including both of these catalyst components in the same catalyst composition, a bimodal product can be produced. The molecular weight modes of the product are intimately mixed, providing a resin product that is relatively free of gels compared to similar products made in staged-reactor processes or by the blending of two distinct unimodal resins.
Controlling the ratio of the components in the bimodal product is a significant manufacturing concern. Product properties of bimodal resins are often sensitive to component split. For instance, in the manufacture of high-density, high-molecular-weight film, to achieve the desired specification requires control of component split within about 2% of the setpoint.
The weight percentage, or “split,” of HMW or high density (“HD”) in the total product including LMW or low density (“LD”) components in a single-reactor manufactured bimodal resin is primarily a function of the relative amount of each type of catalyst in the catalyst system. While theoretically, a catalyst system containing proper amounts of each catalyst could be generated and used to produce the desired split in a particular case, in practice using such a system would be difficult, as the relative productivities of the catalyst components can change with variations in reactor conditions or poison levels.
A technique for changing the flow properties of a bimodal resin is by changing the resin component split, or weight fraction of the HMW component in the product. By modifying the relative amounts of HMW and LMW components in the resin, flow properties can be changed as well. Unfortunately, in some cases changing the split affects more than one variable. In some products, changing the HMW split by a few percent can significantly affect both resin flow index and melt flow ratio (MFR).
One method of changing the component split of a bimodal resin is to add a selective poison (CO2 or H2O) to the reactor. With this method, the flow index is changed by altering the polymer split. Unfortunately, the melt flow ratio (MFR) of the polymer changes with significant flow index variations.
Another example of changing the component split uses a trim catalyst. The primary catalyst is a bimetallic material that makes a bimodal product. To control the split of the polymer, a small amount of low molecular weight producing catalyst is fed independently of the primary catalyst feed; this “trim catalyst” is adjusted to achieve the proper product split. This method controls only the split of the polymer. Flow index and melt flow ratio (MFR) change based on the split change, but independent control of the two is not possible with only the trim catalyst. Moreover, the reactor response times to catalyst changes are relatively long relative to those of reactor gas changes. Changing catalyst composition, because of its low feed rate, is a slower process; replacing the existing catalyst inventory in a reactor with a new blend takes a significant amount of time. Also, feeding a catalyst stream of a pure component to a reaction using bimetallic catalyst likely creates a less homogeneous product mixture. Bimodal resin particles are combined with a small amount of unimodal (usually low molecular weight) resin particles that were made from the trimming catalyst. Unfortunately, low molecular weight particles do not blend well with the lower melt-index bimodal resin particles. This lack of adequate mixing may affect product properties. For instance, while making film from the bimodal resin, inadequately mixed resin particles may result in the formation of gels.
Because it is often desired to control both flow index and melt flow ratio (MFR) in a multi-site polymer process, a technique is needed to enable the independent control of each of these properties.